Method for monitoring oxide film deposition

ABSTRACT

A method for monitoring oxide film deposition is disclosed. The method utilizes silicon wafers having silicon nitride films thereon instead of bare silicon wafers to monitor the growth of silicon oxide/dioxide films in a furnace. The method for monitoring oxide film deposition comprises the following steps. First of all, a wafer having silicon nitride film and a silicon wafer are provided. Next an oxide layer is formed on the wafer and the silicon wafer, and the thickness of the oxide layer is controlled substantially equally on the wafer and the silicon wafer. Then the thickness of the oxide layer on the wafer and the silicon wafer is measured.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for monitoring oxide film deposition, and more particularly to a method for monitoring high temperature oxide (HTO) film deposition in a vertical HTO furnace.

2. Description of the Related Art

Present oxide film deposition processes performed in a vertical furnace have several native issues to be solved. For example, in the oxide deposition process used to form the silicon oxide layers of an oxide-nitride-oxide (ONO) stack layer of a semiconductor device, the electrical thickness of the ONO stack layers formed on wafers on different location of the vertical furnace would vary due to the characteristics of the vertical furnace. The variation of electrical thickness of the ONO stack layer results from the loading of wafer that causes the temperature at the bottom lower than the temperature at the top inside the vertical furnace. To solve this problem, a temperature gradient with high temperature at bottom decreasing toward the top inside the furnace is established in the vertical furnace. Furthermore, the wafer loading sequence between the depositions of the top oxide film and the bottom oxide film of an ONO stack layer is reversed to balance the variation of electrical thickness of the ONO stack layer.

Since the electrical thickness of the ONO stack layer for a semiconductor device such as a flash memory device is a crucial dimension, monitoring of the growth or deposition of the ONO stack layer is inevitable. In order to monitor the deposition of the ONO stack layer, monitor wafers are utilized to find or measure the growth rate of the top and bottom oxide films of the ONO stack layer. However, conventional monitoring ways have a troubling drawback. As shown in FIG. 1, monitor silicon wafers 100 a, 100 b and 100 c having oxide films thereon located on top, middle and bottom inside a furnace respectively are shown. Native oxide films 102 a, 102 b and 102 c are formed on the silicon wafers 100 a, 100 b and 100 c before the wafer boat is loaded in. Since the native oxide films 102 a, 102 b and 102 c are formed before the wafer boat is loaded in, the electrical thickness of these native oxide films are about equal, for example, such as 10 angstroms. However, the electrical thicknesses of native oxide films 104 a, 104 b and 104 c formed during the wafer boat loading in are usually different. For example, the electrical thicknesses of native oxide films 104 a, 104 b and 104 c are 5.3, 4.6 and 3.6 angstroms respectively. The variation of the electrical thicknesses of native oxide films 104 a, 104 b and 104 c generally results from the air flow brought into the furnace during the wafer boat loading and the respective locations or loading distances of the monitor wafers inside the furnace. After the wafer boat is completely loaded in, the furnace is closed and starts to grow oxide films on the silicon wafers 100 a, 100 b and 100 c. Then oxide films 106 a, 106 b and 106 c with different electrical thicknesses are formed on the native oxide films 104 a, 104 b and 104 c. In this circumstance, monitoring of the growth or deposition of the really desired oxide films 106 a, 106 b and 106 c is disturbed by the depositions of the native oxide films. The actual electrical thicknesses of the oxide films 106 a, 106 b and 106 c would not be the predetermined electrical thicknesses. For example, the electrical thicknesses of the oxide films 106 a, 106 b and 106 c are 55.7, 56.4 and 57.4 angstroms as shown in FIG. 1 instead of a constant thickness if a total thickness of 70 angstroms of the oxide films on the silicon wafers 100 a, 100 b and 100 c is measured. This thickness gradient or variation of the deposited oxide layer on the monitor wafer would mislead the setting of deposition process and result in actual thickness of oxide films of a semiconductor device such as a flash memory device deviate the predetermined value and unstable electrical characteristics.

In view of the drawbacks mentioned with the prior art, there is a continued need to develop new and improved method for monitoring oxide film deposition that overcomes the disadvantages associated with prior art. The requirements of this invention are that it solves the problems mentioned above.

BRIEF SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a method for monitoring oxide film deposition to improve the process stability and enhance the yield performance.

It is another object of this invention to provide a method for monitoring oxide film deposition to simplify the production flow.

It is a further object of this invention to provide a method for monitoring oxide film deposition to obtain an actual electrical thickness of the oxide film on a process wafer.

It is a further object of this invention to provide a method for monitoring oxide film deposition to get a constant electrical thickness of the oxide film at different location inside a furnace.

To achieve these objects, and in accordance with the purpose of the invention, the invention provides a method for monitoring oxide film deposition. The method comprises the following steps. At least one silicon monitor wafer having a silicon nitride film thereon is provided. Then the silicon monitor wafer together with at least one silicon process wafer is loaded into a furnace to monitor the growth of an oxide film on the silicon process wafer.

In another embodiment of the invention, the method comprises the following steps. First of all, at least one first silicon monitor wafer having a first silicon nitride film thereon is provided. Then the silicon monitor wafer together with at least one silicon process wafer are loaded into a furnace to monitor the growth of a first oxide film on the silicon process wafer. Next the silicon process wafer and the silicon monitor wafer are removed from the furnace. Then second silicon nitride films are formed on the silicon process wafer. Finally, at least one second silicon monitor wafer together with the silicon process wafer are loaded into the furnace to monitor the growth of a second oxide film on the silicon process wafer.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is best understood by reference to the following detailed description when read in conjunction with the accompanying drawings.

FIG. 1 shows monitor silicon wafers 100 a, 100 b and 100 c having oxide films thereon located on top, middle and bottom inside a furnace respectively; and

FIG. 2 shows monitor silicon wafers 200 a, 200 b and 200 c having silicon nitride and silicon oxide films thereon located on top, middle and bottom inside a furnace respectively.

Common reference numerals are used throughout the drawings and detailed description to indicate like elements.

DETAILED DESCRIPTION OF THE INVENTION

It is to be understood and appreciated that the structures described below do not cover a complete structure. The present invention can be practiced in conjunction with various fabrication techniques that are used in the art, and only so much of the commonly elements are included herein as are necessary to provide an understanding of the present invention.

As shown in FIG. 1 and the description of the background of the invention, native oxide films on bare wafers located inside the top of a furnace would grow faster than native oxide films on bare wafers located inside the bottom of the furnace during the loading process of wafer boat thereby the desired oxide films on the wafers located inside the bottom of the furnace would grow thicker than the oxide films on the wafers located at the top inside the furnace.

In a process of non-volatile memory devices such as flash memory devices, monitor wafers are usually used to monitor the growth rate of oxide films of a silicon oxide-silicon nitride-silicon oxide/oxide-nitride-oxide (ONO) stack. The thickness variation of the native oxide films on the monitor wafer resulting from the loading process of the wafer boat and the nature characteristics of the furnace disturbs and misleads the measurement of the electrical thicknesses of the oxide films formed on process wafers. Since the thickness variation of the native oxide films results from those causes which are hard to avoid, a new way to monitor and control the growth rate of oxide films is provided. According to the characteristics of the silicon oxide/dioxide process, native oxide films are hardly formed on the surface of a nitride film at a temperature over 800° C., and wafers with nitride films instead of bare silicon wafers are used to control and monitor the growth of oxide films on process wafers in a furnace.

As shown in FIG. 2, silicon wafers 200 a, 200 b and 200 c having silicon nitride films 202 a, 202 b and 202 c thereon located on top, middle and bottom inside a furnace respectively are shown. The furnace preferably comprises a vertical high temperature oxidation furnace. The silicon nitride films 202 a, 202 b and 202 c are formed on the silicon wafers 200 a, 200 b and 200 c to prevent the formation of any native oxide film on the silicon wafers 200 a, 200 b and 200 c before the wafer boat is loaded in. The silicon nitride films 202 a, 202 b and 202 c can be formed by many processes known in the art, such as chemical vapor deposition processes. The thicknesses of the silicon nitride films 202 a, 202 b and 202 c are preferably unique for the purpose of monitoring oxide growth. Comparing to the silicon wafers 100 a, 100 b and 100 c, the silicon wafers 200 a, 200 b and 200 c having silicon nitride films 202 a, 202 b and 202 c thereon do not have native oxide films formed before the wafer boat is loaded in. The silicon nitride films 202 a, 202 b and 202 c also prevent any native oxide films formed during the loading process of the wafer boat so that the thickness variations of the native oxide film as well as the following formed silicon oxide films 204 a, 204 b and 204 c are also avoided. Therefore, the thicknesses of the silicon oxide films 204 a, 204 b and 204 c of the monitor silicon wafers 200 a, 200 b and 200 c and the silicon oxide films formed on the process wafers are nearly the same. In an actual process, the silicon wafers 200 a, 200 b and 200 c having silicon nitride films 202 a, 202 b and 202 c thereon and a plurality of silicon process wafers are loaded together into a furnace to form silicon oxide films 204 a, 204 b and 204 c on the silicon wafers 200 a, 200 b and 200 c and the silicon process wafers, and the thickness of the silicon oxide films 204 a, 204 b and 204 c is controlled substantially equally on the silicon wafers 200 a, 200 b and 200 c and the silicon process wafers. Then the thickness of the silicon oxide films 204 a, 204 b and 204 c is measured.

When the silicon wafers having silicon nitride films thereon are used to monitor the growth/deposition of oxide films of a non-volatile memory device such as a flash memory device, the method for monitoring oxide film deposition comprises the following steps. First silicon monitor wafers having first silicon nitride films thereon are loaded together with silicon process wafers into a furnace to deposit first silicon oxide films on the first silicon monitor wafers and the silicon process wafers, and the thickness of the first oxide films are controlled substantially equally on the silicon monitor wafers and the silicon process wafers. After the first silicon oxide films are formed on the silicon process wafers and the silicon monitor wafers, the silicon process wafers and the first silicon monitor wafers are removed from the furnace. Then the thickness of the first oxide films is measured. Then second silicon nitride films are formed on the silicon process wafers. The second silicon nitride films can be formed by chemical vapor deposition processes. Next second silicon monitor wafers having third silicon nitride films thereon together with the silicon process wafers are loaded into the furnace to deposit second silicon oxide films on the silicon process wafers, and the thickness of the second oxide films are controlled substantially equally on the second silicon monitor wafers and the silicon process wafers. Then the thickness of the second silicon oxide films is measured. The first silicon oxide films, second silicon nitride films and the second silicon oxide films on the silicon process wafers are oxide-nitride-oxide stack of the non-volatile memory device such as a flash memory device.

The advantages of the invention includes the followings. First of all, in the process of non-volatile memory devices, the wafer loading sequence between the depositions of the top oxide film and the bottom oxide film of an ONO stack layer is not necessary to be reversed to balance the variation of electrical thickness of the ONO stack layer. The production arrangement involving oxide film deposition can be improved. Moreover, the thicknesses of silicon oxide/dioxide films on the process wafers are equal to the thicknesses of silicon oxide/dioxide films formed on the monitor wafers disregarding the loading process of wafer and the characteristics of a furnace.

Other embodiments of the invention will appear to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples to be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. 

1. A method for monitoring oxide film deposition comprising: providing at least one silicon monitor wafer having a silicon nitride film thereon; and loading said silicon monitor wafer together with at least one silicon process wafer into a furnace to monitor the growth of an oxide film on said silicon process wafer.
 2. The method for monitoring oxide film deposition according to claim 1, wherein three said silicon monitor wafers are loaded together with said silicon process wafers into said furnace and are located at top, middle and bottom inside said furnace respectively.
 3. The method for monitoring oxide film deposition according to claim 1, wherein said furnace comprises a vertical high temperature oxide furnace.
 4. The method for monitoring oxide film deposition according to claim 1, wherein said oxide film grows at a temperature over 800° C.
 5. The method for monitoring oxide film deposition according to claim 1, wherein said oxide film is formed on a nitride film on said silicon process wafer as a top oxide film of an oxide-nitride-oxide stack.
 6. The method for monitoring oxide film deposition according to claim 1, wherein said silicon process wafer is for a process of non-volatile memory device.
 7. The method for monitoring oxide film deposition according to claim 6, wherein said non-volatile memory device comprises a flash memory device.
 8. A method for monitoring oxide film deposition comprising: providing at least one first silicon monitor wafer having a first silicon nitride film thereon; loading said first silicon monitor wafer together with at least one silicon process wafer into a furnace to monitor the growth of a first oxide film on said silicon process wafer; removing said silicon process wafer and said first silicon monitor wafer from said furnace; forming second silicon nitride film on said silicon process wafer; and loading at least one second silicon monitor wafer having a third silicon nitride film thereon together with said silicon process wafer into said furnace to monitor the growth of a second oxide film on said silicon process wafer.
 9. The method for monitoring oxide film deposition according to claim 8, wherein three said first silicon monitor wafers are loaded together with said silicon process wafers into said furnace and are located at top, middle and bottom inside said furnace respectively.
 10. The method for monitoring oxide film deposition according to claim 8, wherein said furnace comprises a vertical high temperature oxide furnace.
 11. The method for monitoring oxide film deposition according to claim 8, wherein said oxide film grows at a temperature over 800° C.
 12. The method for monitoring oxide film deposition according to claim 8, wherein said silicon process wafer is for a process of non-volatile memory device.
 13. The method for monitoring oxide film deposition according to claim 12, wherein said non-volatile memory device comprises a flash memory device.
 14. The method for monitoring oxide film deposition according to claim 8, wherein three said second silicon monitor wafers are loaded together with said silicon process wafers into said furnace and are located at top, middle and bottom inside said furnace respectively.
 15. A method for monitoring oxide film deposition comprising: providing a wafer having silicon nitride film and a silicon wafer; depositing an oxide layer on the wafer and the silicon wafer, and controlling the thickness of the oxide layer substantially equally on the wafer and the silicon wafer; and measuring the thickness of the oxide layer on the wafer and the silicon wafer.
 16. The method for monitoring oxide film deposition according to claim 15, wherein the oxide film grows at a temperature over 800° C.
 17. The method for monitoring oxide film deposition according to claim 15, wherein the oxide film comprises a high temperature oxide film.
 18. The method for monitoring oxide film deposition according to claim 15, wherein said oxide layer is deposited in a vertical high temperature oxidation furnace. 